1. Field of the Invention
The present invention relates to an organic electroluminescent display device and a method of driving the same, and more particularly, to an organic electroluminescent display device which can effectively prevent voltage drop and ensure a simple layout, and a method of driving the organic electroluminescent display device.
2. Description of the Related Technology
FIG. 1 is a block diagram of a conventional organic electroluminescent display device 100. Referring to FIG. 1, the organic electroluminescent display device 100 includes a data driver 110, a scan driver 120, and a display unit 130. The display unit 130 includes a plurality of data signal lines which are arranged in a vertical direction, and a plurality of select signal lines which are arranged in a horizontal direction.
In the display unit 130 of the organic electroluminescent display device 100, pixels are defined in the form of a matrix by the data signal lines and the select signal lines, and pixel circuits are arranged in the pixel region.
The data driver 110 transmits data signals D[1] through D[m] for controlling the luminous intensity through the data signal lines to the display unit 130. The scan driver 120 applies scan signals S[1] through S[n] through the scan signal lines to select a line of pixels constituting the display unit 130. Information on the data signals D[1] through D[m] is transmitted to the line of pixels selected by the scan signals S[1] through S[n]. A first voltage source supplies a constant high power supply voltage VDD to all the pixels of the display unit 130.
FIG. 2 is a circuit diagram of a pixel circuit employed by the conventional organic electroluminescent display device of FIG. 1.
Referring to FIG. 2, the pixel circuit employed by the conventional organic electroluminescent display device includes an organic electroluminescent device (OLED), two transistors (M1, M2), and one capacitor Cst. One of the two transistors is a switching transistor M1, and the other transistor is a driving transistor M2. The number and interconnection of the transistors and the capacitor of the pixel circuit may be changed according to necessary operations of the electroluminescent display device. The transistors are generally thin film transistors (TFTs).
Referring to FIG. 2, a first electrode of the switching transistor M1 is connected to a data line. When the switching transistor M1 is turned on by a scan signal applied to its gate electrode, a data signal (D[m]) is applied into the pixel circuit due to the switching operation.
The capacitor Cst is connected between a first electrode and a gate electrode of the driving transistor M2 to maintain a data voltage applied through the switching transistor M1 for a predetermined period of time. Also, the driving transistor M2 supplies a current corresponding to the voltage between both terminals of the capacitor Cst to the OLED.
When the switching transistor M1 is turned on, a data voltage applied through the data line is stored in the capacitor Cst, and when the switching transistor M1 is turned off later, a current corresponding to the data voltage stored in the capacitor Cst is applied to the OLED through the driving transistor M2, so as to emit light.
The current flowing through the OLED is given by the following formula.
                                                                        I                OLED                            =                                                β                  2                                ⁢                                                                  ⁢                                                      (                                                                  V                        gs                                            -                                              V                        th                                                              )                                    2                                                                                                        =                                                β                  2                                ⁢                                                                  ⁢                                                      (                                                                  V                        DD                                            -                                              V                        data                                            -                                                                                                V                          th                                                                                                              )                                    2                                                                                        (        1        )            where IOLED denotes a current flowing in the OLED, Vgs denotes a voltage between a gate and a source of the driving transistor M2, Vth denotes a threshold voltage of the driving transistor M2, VDD denotes a first power supply voltage, Vdata denotes a data voltage, and β denotes a gain factor.
Since the conventional organic electroluminescent display device 100 undergoes a voltage drop due to a first voltage line through which the first power supply voltage VDD is applied, the value of the first power supply voltage VDD applied to the plurality of pixels is not constant.
As shown in FIG. 2, the current applied to the OLED is greatly dependent on the magnitude of the first power supply voltage VDD. Accordingly, when the first power supply voltage VDD drops, a desired amount of current does not flow through the OLED for each pixel, thereby degrading image quality. The voltage drop problem becomes worse as the size of the display unit 130 increases and brightness increases.
If a separate circuit is installed to solve the image quality degradation due to the voltage drop, an aperture ratio of the panel layout decreases, thereby degrading brightness.